The present invention relates to a handle-shaped molded article having the form of a handle, a drawer pull, a knob, or the like (to be referred to as "handle" hereinafter) used for opening and closing the door, etc., of an automobile, an electric appliance, a building, and the like, a method of injection molding of the handle-shaped article, and a mold suitable for molding the handle-shaped molded article. More specifically, it relates to a handle-shaped molded article of which the grip portion has an excellent appearance and the fitting portion has high strength, a method of injection molding the same, and a mold suitable for molding such a handle-shaped molded article.
A handle-shaped part having a thick grip portion and thick fitting portion(s) is fixed to a door, etc., of an automobile, an electric appliance or a building, and is used to open the door. Such a handle-shaped part is required to be light in weight as a whole. The grip portion is evaluated on its appearance, and is required to be free from a sink mark and distortion. Further, the fitting portion of the handle-shaped part is required to have high strength.
A thick part can be generally produced by a foam molding method. When a handle-shaped part is produced by the foam molding method, however, cellules remain on the surface of the handle-shaped part, and it is therefore difficult to produce an excellent appearance. Further, since cells remain in the interior of the fitting portion, the strength of the fitting portion is insufficient. Furthermore, in the foam molding method, when a molten resin and a foam are mixed, a long period of time is required for cooling, and the molding cycle takes longer.
On the other hand, there is known another foaming technique in which a molded article is obtained by first injecting a molten resin containing no foaming agent into a cavity provided in a mold and then injecting a molten resin containing a foaming agent into the cavity. A molded article produced by this technique has an excellent appearance, since the surface layer of the molded article is formed from a resin containing no foaming agent. However, in the production of a handle-shaped part by such a technique, the structure of the injection molding apparatus and the molding conditions are complicated, and the molding cycle also takes longer.
U.S. Pat. No. 4,101,617 or JP-B-57-14968 discloses a method in which a pressurized gas is applied to a molten resin injected into a cavity provided in a mold to produce an injection molded article having a hollow structure. Further, JP-B-61-53208 discloses an injection molded article having a hollow portion extending non-uniformly from that portion of the molded article which corresponds to a resin gate, and a method in which such an injection molded article is produced.
In the method of any one of U.S. Pat. No. 4,101,617, JP-B-57-14968 and JP-B-61-53208, a gas inlet is provided in a resin nozzle portion. Therefore, when a resin gate is provided in that portion of the mold where the fitting portion of a handle-shaped molded article is to be molded and the handle-shaped molded article is produced according to these methods by using such a mold, it is difficult to form a fitting portion having a solid structure, and the fitting portion cannot exhibit high strength. Further, when a resin gate is provided in that portion of the mold where the grip portion of the handle-shaped molded article is to be molded, the hollow portion of the grip portion extends toward the fitting portion although the extension depends upon the shape of the handle-shaped molded article, and it is difficult to accurately form a solid structure of the fitting portion.
In the above method of any one of the above three publications, after a molten resin in an amount insufficient to fill a cavity is injected, or while a molten resin is being filled into a cavity, a pressurized gas is introduced into the cavity through the gas inlet provided in the resin nozzle portion, and then the molten resin is cooled and solidified. However, nothing is described or suggested concerning the relationship between the resin pressure and the gas pressure while the molten resin is cooled and solidified.
JP-A-60-24913 (corresponding to U.S. Pat. No. 4,824,732) and JP-A-64-14012 (corresponding to U.S. Pat. No. 4,740,150) describe a method in which an injection molded article having a hollow structure is produced by applying a pressurized gas from the center of a cavity to a molten resin, an injection molding apparatus used therefor.
In one embodiment of JP-A-60-24913, as soon as a molten resin introduced into a cavity passes the outlet of a gas passage and closes the outlet, a pressurized gas is introduced into the molten resin from the gas passage, whereby a hollow portion is formed in the molded article. In this embodiment, however, it is very difficult to form a handle-shaped molded article having a hollow structure. That is because the pressurized gas, which has been introduced into the molten resin through the outlet of the gas passage as soon as the molten resin has closed the outlet of the gas passage, blows away the molten resin which has been introduced to that portion of the cavity where a grip portion having a large cross section is to be formed.
In one embodiment of JP-A-24913, a screw ram of an injection molding machine is held on in a forward end of a stroke while a molten resin is cooled and solidified. However, JP-A-24913 neither describes nor suggests anything concerning the relationship between the pressure for controlling the screw ram and the pressure of a pressurized gas. If the screw ram is allowed to stay in a forward end without applying any pressure to the screw ram and a pressurized gas is introduced into a molten resin introduced into a cavity, the molten resin which has been introduced into the cavity is pushed back into a resin gate by the pressurized gas. As a result, not only a grip portion becomes hollow, but also the molten resin is sometimes pushed back into the resin nozzle portion of an injection molding apparatus, thereby pushing back the screw ram.
JP-A-14012 discloses a method in which a sliding valve provided in a resin nozzle portion of the injection molding apparatus is closed. This method seems to prevent the counterflow of a molten resin injected into a cavity toward a resin gate and the formation of a hollow portion within a fitting portion. When the sprue portion and the runner portion have a large volume, however, a molten resin present in the sprue portion and the runner portion contract during its cooling and solidification, and a hollow portion is likely to be formed in the fitting portion due to the pressurized gas. Even the sprue portion and the runner portion are liable to become hollow in some cases. As a result, the resultant fitting portion has insufficient strength.
JP-A-268611 (corresponding to U.S. Pat. No. 4,923,666) discloses a method in which an injection molded article having a hollow structure is produced by introducing a pressurized gas into a molten resin. In this method, a molten resin is injected into a cavity in such an amount that it occupies the entire volume of the cavity. It is therefore difficult to apply this method to the production of a handle-shaped molded article having a thick grip portion and a large-volume hollow portion.